There is a conventionally known electron tube used to detect the presence or absence of a flame on the basis of ultraviolet rays emitted from a flame in a combustion furnace or the like. The electron tube includes a sealed container in which predetermined gas is filled in a sealing manner, two electrode supporting pins that penetrate through both end portions of the sealed container, and two electrodes (a pair of electrodes) that are supported in parallel with each other by the electrode supporting pins within the sealed container.
In the electron tube, when one electrode facing a flame is irradiated with ultraviolet rays in a state where a predetermined voltage is applied across the electrodes through the electrode supporting pins, electrons are emitted from the one electrode due to the photoelectric effect and excited in succession one after another to cause an electron avalanche between the one electrode and the other electrode. Therefore, it is possible to detect the presence or absence of a flame by measuring a change in impedance between the electrodes, a change in voltage between the electrodes, a change in current flowing between the electrodes, and the like. Various methods for detecting the presence or absence of a flame have been suggested (see PTL 1 and PTL 2, for example).